1. Field of the Invention
The invention in general relates to detection systems, and more particularly to a system for determining the location of persons and objects via acoustic sensors.
2. Background
A variety of methods and systems exist for surveillance around buildings and ground assets. One method used to protect the perimeter around a critical facility from stealthy intruders is to post sentries and trip-wire alarms, or to instrument critical areas with video cameras and have guards monitor the screens. Both of these methods are too human intensive for a large perimeter, and system performance is vulnerable to fatigue by the guards faced with a tedious task. The modern trend is toward remote sensing of the entire perimeter to provide automated alerts to which sentries can then be directed, and otherwise complement the performance of human assets. Further, remote sensing coupled to automated alerts can lower the cost to defend a perimeter by lowering the number of human sentries required.
In general, a single sensor type does not provide sufficient detection coverage for the full variety of expected targets. For example, a quiet stealthy intruder on foot is more difficult for a passive acoustic sensor to detect than it is for an IR sensor. Several current intrusion alert systems use video, radar, infra-red, and passive acoustic sensors. All have varying degrees of problems with excessive numbers of false alarms. Specific weaknesses associated with these systems, in addition to the high false alarm rates, are very limited detection ranges (especially for stealthy intruders), and a lack of automatic threat localization and tracking. Thus these systems could be enhanced with an additional sensor type that can provide automatic detection coverage for quiet slowly moving intruders out to about two hundred feet and that can help resolve true from false targets among the multiple reports.
Perimeter surveillance systems may use a passive acoustic capability, in the band of human hearing, to provide covert sensing, target signature recognition, lines of bearing to contacts and effective information presentation to an operator. Complete dependence on passive acoustics, however, places some limitations on overall system performance. Passive sensing alone will not provide adequate detection and localization performance against a stealthy human intruder, as they move slowly, on foot, and are trained to be virtually undetectable.
In other fields, active ultrasonic acoustic systems have been developed which provide short-range measuring applications such as airbag systems, construction measurements, proximity warning systems, body imaging, and robotics, such as described in U.S. Pat. No. 5,577,006 to Kuc and U.S. Pat. No. 6,268,803 to Gunderson et al. Most of these acoustic applications are intended for indoor use and operate at ranges less than 20 feet. None of the previous applications of the ultrasonic acoustic detections in air were concerned with extending the operating range of the system for identifying targets to several hundred feet, nor do they use complex transmit waveforms, array processing or sophisticated signal processing techniques to improve signal to noise ratio (SNR) and thus increase the range at which the active echo could be detected. While long range active sonar array systems are known for underwater applications, similar approaches have not been introduced into air-acoustic systems because of, among other reasons, the different characteristics of air versus underwater acoustic environments.
Thus, to date the systems based on air-acoustic detection and characterization either rely on limited passive systems at human audio frequencies, or are directed at limited ultrasonic applications like short-range distance measurement or imaging, and have not attempted to address issues like long-range detection using active air-acoustic systems at ultrasonic frequencies.